Field of the Invention
The present invention relates to an image capture apparatus and control method thereof and, more particularly, to an image capture apparatus which can capture a moving image, a dynamic range of which is expanded, and a control method thereof.
Description of the Related Art
Conventionally, an image sensor (CCD image sensor, CMOS image sensor, or the like) used in a general digital camera suffers a problem of a narrower dynamic range (an input luminance range expressible by output values) than a silver halide film. As a method of obtaining an image having a dynamic range broader than that of the image sensor, a technique called “HDR (High Dynamic Range)” is known. The HDR is a method of generating one image (HDR image) by compositing a plurality of images (for example, an overexposure image, underexposure image, and the like) which are obtained by capturing a single scene using different exposure amounts (Japanese Patent Laid-Open No. 11-164195).
Also, in recent years, a technique for generating an HDR image using an image sensor, which can capture a plurality of images having different exposure amounts in an image capture operation for one frame period, has been proposed, and application to HDR image generation in a moving image capture operation has also been referred to (Japanese Patent Laid-Open No. 2011-244309).
In an HDR moving image capture operation for generating a frame image by compositing a plurality of images, when evaluation vales relating to image capture, which are required for image capture are detected from a composite image, a reflection timing of the evaluation values relating to image capture is delayed. This problem is posed in the arrangement which acquires a plurality of images having different exposure amounts for a plurality of frame periods, as described in Japanese Patent Laid-Open No. 11-164195, and also in the arrangement which can acquire a plurality of images having different exposure amounts within one frame period, as described in Japanese Patent Laid-Open No. 2011-244309. Note that the evaluation values relating to image capture include an AF (Auto Focus detection) evaluation value, AE (Auto Exposure control) evaluation value, WB (White Balance) evaluation value, and the like.
For example, a case will be examined wherein an AF control operation is executed based on an AF evaluation value during an image capture operation of an HDR moving image. In this case, when an arrangement for executing the AF control operation using the AF evaluation value detected from a composite image is adopted, a reflection timing of the AF evaluation value comes after image composition, resulting in poor AF response to motions of a subject. For this reason, it is desired to detect and reflect the AF evaluation value using images before composition.
However, in an arrangement which detects evaluation values relating to image capture always using one of images before composition (for example, an overexposure image having a large exposure amount and an underexposure image having a small exposure amount), accurate evaluation values relating to image capture cannot often be detected due to the relation between the exposure amount and image capture scene. For example, in a very dark scene in which an image of a person is captured with the night view in the background, accurate evaluation values relating to image capture cannot be detected from the underexposure image. Alternatively, when a subject suffers a shadow-detail loss or highlight-detail loss since an image capture scene is sufficiently bright and has a broad dynamic range, accurate evaluation values relating to image capture cannot be detected from the overexposure image.
The same problem is posed in a backlit scene (in which the background of a principal subject is very bright), a tunnel scene (in which the background is dark, and an occupation area of a principal subject is small), and the like.